Armature with automatic weights is deforming mesh
Mesh normals are inverted which is the main cause of the issue.

To correct it, enter edit mode, select all and use ShiftN to recalculate normals.
Also the mesh is scaled and rotated. Preferably, apply scales and rotations before parenting CtrlA in object mode.
Last point, the armature is not in rest position: set it to it or set the current pose as rest pose.